1. Field of the Invention
The present invention relates to a processing liquid and a processing method for a lithographic printing plate precursor. The invention particularly relates to a processing liquid and a processing method for a lithographic printing plate precursor having an image recording layer that can be removed with at least one of printing ink and dampening water.
2. Description of the Related Art
In a plate manufacturing step of a lithographic printing plate precursor, there is suggested a method called an on-press development that obtains a lithographic printing plate by performing image-exposure by using an image recording layer in which an unnecessary portion of a lithographic printing plate precursor can be removed in a general printing step and removing a non-image portion from a printing press. According to this method, it is possible to obtain a lithographic printing plate by electronically processing image information using a computer, accumulating the image information, causing a highly astringent radiation ray such as a laser beam to carry the digitalized image information according to outputting digitalizing techniques, performing image-exposure due to scanning exposure of the lithographic printing plate precursor, and removing a non-image portion using at least one of printing ink and dampening water from a printing press. Therefore, it is possible to omit a development processing step of a lithographic printing plate, which is a necessary step in the printing field in the related art. Accordingly, a plate manufacturing operation can be performed in a completely dry circumstance (in which liquid is not used), such that operational and environmental burdens can be greatly reduced.
The plate manufacturing by the on-press development has advantages as described above, but the nature of having a step of removing an image recording layer by using a printing press, in view of preventing coloring contaminations of ink and dampening water, only a minimum amount of colorant can be used in the image recording layer, and this may cause a problem. Generally, as a step performed before a printing plate is installed on a printing press, a plate inspection operation of inspecting and identifying an image on a printing plate, such as whether the image is recorded on the printing plate as desired and what color of ink for the plate is, is performed. In the general lithographic printing plate precursor accompanied by the development processing step using a developing liquid, it is easy to check an image after the development process and before the printing (before the printing plate is installed in the printing press), by adding a colorant in the image recording layer. However, in the case of an on-press development-type lithographic printing plate precursor that is not accompanied by the development processing step before printing, the image recording layer contains only a small amount of colorant, and thus, in a step in which the lithographic printing plate precursor is installed in the printing press after the exposure, it is difficult to check an image and it is not possible to identify a printing plate. Therefore, this causes an erroneous operation.
In addition, in the field of printing, an operation of checking whether appropriate exposure and development have been performed by using a halftone dot area measuring machine or a concentration meter in the related art has been generally performed. The operation is called a calibration operation, and is an important operation that is continuously performed in order to properly maintain image quality all the time. There are plural kinds of methods of checking the image quality, but a method of outputting an image in a predetermined halftone dot area ratio with an exposure machine, performing a development process, measuring and digitizing obtained halftone dots by using a halftone dot area measuring machine or a concentration meter, and changing an exposure signal if a numerical value is deviated such that the numerical value comes close to a desired numerical value is generally employed. Here, the concentration meter used herein has detection sensitivity in a general visible range (400 nm to 700 nm). Therefore, in the case of the on-press development-type lithographic printing plate which has little absorption in this wavelength range as described above, an image is not sufficiently detected and thus a calibration operation may not be sufficiently performed. In addition, the halftone dot area measuring machine obtains halftone dots as an image with a device such as a CCD, performs image-processing with the obtained image, divides the image into a non-image portion and an image portion using a certain threshold value, and digitizes an area ratio of the image portion. Therefore, if a contrast between the non-image portion and the image portion is low, the image is not sufficiently detected, and thus the calibration operation may not be sufficiently performed.
Further, recently, requests for a high-definition printed matter has been increasing, and according to these requests, a high definition image is frequently formed by exposing the image with a high-definition AM screen or an FM screen. However, the high-definition image is formed with an aggregation of finer dots (halftone dots), and thus, due to poor development or ununiform development of an unexposed region, and unevenness of screen tint or in-plane non-uniformity of halftone dot sizes on a printing plate that have not been not generated when an AM screen in the related art is used is generated such that halftone dot reproducibility decreases. In order to deal with these problems, it is required to more accurately identify an image portion and a non-image portion in plate inspection or image quality control.
As a method of plate inspection or image quality control in an on-press development-type lithographic printing plate precursor, JP2007-230185A discloses a processing liquid including a basic coloring dye and an anionic surfactant and a method of processing an on-press development-type lithographic printing plate precursor by using the processing liquid. In addition, JP2012-529668A discloses a method of manufacturing a lithographic printing plate including a step of coating a coloring liquid including a water-insoluble colorant and a specific solvent.
However, according to the techniques disclosed in the patent documents described above, plate inspection or image quality control of an on-press development-type lithographic printing plate precursor may not be sufficiently performed. Particularly, if a high definition image is formed by performing exposure using a high definition AM screen or a high definition FM screen, it is not substantially possible to perform plate inspection or image quality control.